1. Field
This invention relates generally to porous glass particles and specifically to an aqueous suspension of porous glass particles having a closely controlled average particle size, the suspension being especially useful in the field of solid phase radioimmunoassay.
2. Prior Art
Porous glass refers to glass which contains an intricate network of minute interconnected voids and channels. Such glass is made by first forming a body of phase-separable glass within a certain compositional range, subjecting the formed glass to a controlled heat treatment to separate it into two phases, only one of which is acid soluble, and subsequently dissolving out the soluble phase with appropriate reagents to produce the void space within the glass. The average pore diameter of the interconnected channels is commonly very small and measured in Angstrom Units (A). The ultimate pore size depends on such factors as initial glass composition, heat treatment duration and temperature, and soluble-phase separation techniques. The actual preparation of porous glass bodies is described more fully in U.S. Pat. No. 2,106,744, issued to Hood and Nordberg. Various techniques for enlarging the pores of porous glass are described in U.S. Pat. No. 3,485,687, issued to Chapman et al and U.S. Pat. No. 3,549,524, issued to Haller. A technique for preparing porous glass particles having two essentially distinct zones of porosity is disclosed in U.S. Pat. No. 3,790,475. By following specific teachings of the above disclosures, it is possible to closely control the average pore diameter of the porous glass bodies such that (e.g. U.S. Pat. No. 3,549,524), as much as 95% of the total pore volume consists of "pores" having an average pore diameter within .+-.20% or better. Such glass is commonly referred to as controlled pore glass or, simply, CPG. CPG can be readily prepared having an average pore diameter within the range of about 30 A to about 2500 A and is available commercially in a form having a variety of average pore diameters. The particle sizes of such porous glasses is generally in the range of about 40 to 400 mesh, U.S. Standard Sieve.
Most porous glass consists of at least about 96% silica, about 3% B.sub.2 O.sub.3, and small amounts of other ingredients (e.g. Corning Code 7403, 7417, or 7930 porous glass). Because of many desirable physical and chemical properties (inertness, relatively small average pore diameter, high surface area per gram, and chemical composition), porous glass has been found useful in chromotographic applications (e.g. U.S. Pat. No. 3,114,692) and, more recently, as a carrier material for such biologically active materials as enzymes (e.g. U.S. Pat. No. 3,519,538) and antibodies (e.g. U.S. Pat. No. 3,652,761). In U.S. Pat. No. 3,652,761, there are disclosed various techniques for chemically coupling specific antibodies through intermediate silane coupling agents to porous glass particles in such a manner that the attached antibody retains its ability to complex with a specific antigen. Thus, by reacting such immobilized antibodies with a solution containing antigens specific to the antibody, it has been found possible to isolate or separate a given antigen from a solution containing other substances. Inasmuch as the complexing of a given antigen with an antibody to that antigen is a very specific reaction, the use of an immobilized antibody permits the extraction of substances having a very low concentration.
In recent years, there has been a growing recognition of the need to know concentrations of various substances which often exist in quantities as low as nanograms per ml of a solution. Since such concentrations cannot be readily determined via classical analytical methods, there has been developed a relatively new analytical method referred to as radioimmunoassay (RIA). RIA is a term used to describe a method of determining very low concentrations of substances which method is based on the use of radioactively labelled materials which can form immunochemical complexes. The RIA of a given substance for which there exists antibodies is based on the observation that an unknown amount of the given substance (unlabelled) will tend to compete equally with a known amount of that substance (labelled) for a limited number of complexing sites on antibodies specific to that substance to form immunochemical complexes of both antibody-substance (inlabelled) and antibody-substance (labelled). By separating the complexed products from the reaction solution, and then counting the radioactivity of either the separated complexes or the remaining solution, it is possible to determine the unknown concentration by relating the count to a standard curve prepared beforehand using known amounts of labelled substance.
An essential step in RIA involves separating the complexed products from the reaction solution. To facilitate the separation, it has been found highly desirable to use composites consisting of antibodies which have been immobilized on essentially water insoluble carrier materials. The use of such carriers in RIA has come to be known as solid-phase RIA or, simply, SPRIA.
The immunochemical composites described in U.S. Pat. No. 3,652,761 have been suggested as possibly useful for SPRIA. However, in attempting to use the teachings of that patent to prepare composites for SPRIA, it was found that, in many cases, it was difficult to achieve concentration measurement sensitivity in clinically significant concentration ranges. To a certain extent, that finding was surprising since some of the carriers disclosed therein (e.g. porous glass, about 500-600 A average pore diameter) had relatively high surface areas per gram, thus assuring the loading of relatively large amounts of antibody which should have permitted a fairly high degree of sensitivity. It was subsequently found that carrier particle size plays an important role in allowing a highly sensitive SPRIA and we have been able to prepare a suspension of suspendable porous glass particles having a very closely controlled particle size range which is ideally suited for use in SPRIA.